Patent GB-945381-A (GB)
Title: Traffic monitoring and control system
IPC: G08G1/01
Filed: 1960-01-21
Abstract: 945,381. Road signals for controlling traffic. LABORATORY FOR ELECTRONICS Inc. Jan. 21, 1960 [Feb. 5, 1959], No. 2307/60. Heading G4Q. In a traffic monitoring and control system means are provided for determining the volume and speed of road traffic and for determining concurrent changes in volume and speed, indicators of road traffic signals being operated when the rates of change of volume and speed both exceed predetermined limits. Fig. 1 represents a section of a motorway, on which traffic is proceeding in the direction of the arrow 13. A vehicle passage and speed sensing detector 17, which may be a Radar Speed and Impulse Unit such as the one described in Specification 929, 631, conveys information of traffic flow via two wire line 22 to monitoring and control centre 23. An output signal which is derived from the information processed in the control centre 23 and which may correspond for example to concurrent decreases in the rates of change of volume and speed, can then be used to operate signals 25 or 27 or the like. Monitoring and control centre, (Figs. 2a and 2b). The traffic flow information on line 22 is separated at "speed and volume impulse translator" 31 into its constituent components of a speed signal and a volume signal. The "average computer" 34 determines the average speed from the speed signals produced by the n cars passing detector 17 in a certain time, volume average computer 35 determining the average number of cars passing per unit time. The details of a suitable "translator" and "average computer" are given in Specification 929, 631. The average speed signal, which is a varying D.C. voltage is smoothed by an RC network to eliminate transients, differentiated in a conventional CR differentiating network in circuit 38 and then applied to a balanced twin triode voltmeter 43 which indicates rate of change of speed as a percentage rate of change per unit time. A range adjustor 45 calibrated in MPH/min is available for making the 100% indication on meter 43 equivalent to a predetermined rate of change of speed e.g. 10 MPH/min. The differentiated signal is applied through a diode and condenser in series to the control grid of the first valve of a high gain amplifier in negative change coincidence unit 52. A further diode is connected between the junction of the first diode and the condenser and the slider of a potentiometer (limit adjust 54) so that if the potential at the slider is lower than that at the junction there is a low impedance path to earth from the control grid and a low A.C. voltage which is permanently connected to the grid will not be amplified. The average volume signal is treated in exactly the same way. If now both rate of change of volume and the rate of change of speed of the traffic fall then the voltages at the diode condenser junctions will fall below those at the sliders, a high impedance path to earth will be presented to the control grid and the valve will amplifythe A.C. voltage. The amplified voltage is detected and used to bias a normally conducting valve beyond cut-off, so that a relay VSL in its anode circuit becomes de-energized. An alarm circuit is thus closed and "instant alarm" 57 which may be on the control panel (Fig. 4 not shown) is operated. Current is also applied by relay VSL to the condenser in a variable RC delay circuit 58 connected to a grid of a twin-triode also in the delay circuit. If the rate of decrease of traffic flow is prolonged, the voltage on the condenser can build up sufficiently to allow the valve to conduct and so operate a self locking relay in the "Hold/Release selector" 62, with consequent operation of "delayed alarm" 63. The instant, delayed, and hold-release, outputs on lines 56<SP>1</SP>, 60, 66<SP>1</SP> respectively can be used to operate remote indicators such as 25, 27 in Fig. 2c. The selector 62 may be released manually or by a signal from the positive change coincidence unit, 53. In unit 53, the polarity of the diodes is reversed so that only concurrent increases in the rates of change of speed and volume (beyond a predetermined amount) will allow the A.C. voltage to be amplified, the output of this unit then operates further signals and/or cancels the signals set up by the negative change coincidence unit. A circuit diagram of Fig. 2b is given in Fig. 3 (not shown). Specification 941, 921 also is referred to.